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Aviation History
1961
1961 - 0045.PDF
FLIGHT, 13 January 1961 43 HPOUHESTOS fig 4 Tubular-extension nozzle (below) Fig 5 Ball/socket nozzle (right) Fig 6 Rotating nozzle (far right) CARBON DURESTOS H P DuuESTOS. DURESTOS HiaH PBeSSuRE DuRESTOS STEEL BAL DIA 55 O CARBON INSERT- iSERT BALL BETXINING PLUG "HISH PRES; DORE5TOS 1OLV8DENUM MOLYBDENUM a torque of about 1201b-in. Larger motors require swivellingnozzles, and a ball/socket nozzle (Fig 5) uses a similar O-ring, sliding on a polished steel surface. At a pressure of 5001b/sq in,and thrust of 7,0001b, a maximum torque of 2001b-in is required to move the nozzle ±10° at 0.5c/s; aggregate weight of theassembly is 171b. The final nozzle illustrated (Fig 6) has a rotating portion moulded in Durestos, with a steel insert actingas the inner race of an angular-contact ball bearing. This nozzle must be rotated ±30° to produce a deflection of ±10°, and at0.5c/s a torque of some l,5001b-in is required for a 7,0001b-thrust nozzle operating at 50Olb/sq in. Total weight is 141b. Extraweight for nozzles fitted to a light end-closure with four reinforced apertures would be about 81b per nozzle for the ball/socket and61b for the rotating pattern. These figures must be increased by about 30 per cent for nozzles matched to vacuum operation. Dual-thrust Motors The second paper from Summerfield, Solid-Propellant Dual-Thrust Motors, by H. M. Darwell, outlined the factors affecting the choice of propulsion system for missiles, many of which havelead to an infinite variety of dual-thrust systems. Broadly, dual- thrust motors can be grouped into four categories : single chamber,fixed nozzle; single chamber, variable nozzle; double chamber, single external nozzle; and tandem double chamber, with transfertube from the forward chamber. One of the neatest ways of changing combustion pressure is toemploy a first-stage propellant separated from a second stage, with a different burning rate, by an interface corresponding to aburning surface. Motors of this type are essentially simple, having a single body and charge of conventional configuration anda single, fixed nozzle. Another solution is to introduce a change into the burning surface in a single propellant; but the fact thatthe motor has to burn at two widely different pressures can lead to excessive variation between one motor and the next. Combin-ing the two methods can lead to a wide variety of motors with two different propellants with different burning rates and burningsurfaces. At this point Mr Darwell included a plot of single- chamber firings, showing that thrust ratio, between 2 and about8.4, was directly proportional to reduced pressure ratio. Specific impulses of such motors are currently about 150sec and there areindications that this level will in the near future be substantially increased. Probably the ideal arrangement is to employ a single-chambermotor with a two-setting nozzle, throat area being decreased to correspond with the lower mass flow in the second stage of opera-tion. So far no satisfactory design is known to have been evolved, but a promising arrangement involves an ejectable first-stagenozzle (Fig 7). The chief difficulties appear to be control of the rate of changeover and distortion due to heating. Dual-chamber motors with a single external nozzle include aninternal choke to raise the pressure in the forward chamber after consumption of the boost charge, and so allow the second-stagegases to pass at a reduced rate through the rear chamber and nozzle. Numerous experimental firings have confirmed thetheoretical explanation ot how such motors function. A major advantage of the arrangement is that there is no practical limitupon thrust ratio, and values as high as 50:1 have been achieved; but it is essential to insulate the rear chamber adequately, and athigh thrust ratios the specific impulse from the second stage is poor at low altitudes. At present the most satisfactory means ofobraining thrust ratios greater than 8:1 at low altitudes is the double-chamber motor with a transfer tube [such as that illustratedin the drawing on page 85 of our July 15, 1960, issue—Ed]. Specific impulses of 150sec are being achieved. In conclusion, Mr Darwell listed some of the actual motorsdeveloped at SRS. Models have been produced for anti-tank, surface-to-air and surface-to-surface bombardment weapons,having propellant weights from 4 to 2,2001b, diameters from 4 to 24in and thrust ratios up to 6:1. Flight trials of single-chamber,single-charge, fixed-nozzle motors are recording a specific impulse of 150. Flight trials are also in hand on a transfer-tube motorof 7.5in diameter, with a thrust ratio of 15:1; despite its small lengrh/diameter ratio and intricate design, it is achieving aperformance index in the region of 120. Fig 7 A nozzle system (right) giving two levels of thrust; A , second-stage nozzle; B, first- stage nozzle; C, release pin; D, explosive charge; E, coil spring NEW US NUCLEAR-ROCKET ESTABLISHMENTA study of the requirements for a national nuclear rocket-engine development facility in the USA is to be made by a joint industryteam, under contract to the National Aeronautics and Space Administration. The proposal which has been accepted was sub-mitted by the Ralph M. Parsons Co in association with the Thiokol Chemical Corporation. In this, Thiokol is representing Talant,a team comprising Thiokol, the Allison Division of General Motors, Linde (a division of Union Carbide Corp) and NuclearDevelopment Corp of America. The architectural and eneineerine study will be under thedirection of the joint NASA/Atomic Energv Commission Nuclear Propulsion Office, which has the responsibility for deyelooingnuclear propulsion systems for space vehicles. Factors involved will be site considerations, number and type of test stands needed, control centre, support facilities, assembly and disassembly build- ings, water and electrical power requirements, construction costs and schedules. SOUTH AFRICAN RADIO TELESCOPE TAKES SHAPE Erection of the 85ft diameter radio telescope located nearKrugersdorp, South Africa, is due to begin this month. Site excavations began last November, and equipment was expected tobegin to arrive from the USA in December. Under the agreement signed between the US National Aeronautics and SpaceAdministration and the South African Council for Scientific and Industrial Research, the station will be operated and maintainedby CSIR personnel. Total cost is quoted as between £lm and £2m.
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